Hi Dario,
it is difficult to answer for me as I have never seen a MRT QUAD nas from the inside.
Hopefully you get an answer at MRT forum.
Edit: found a guide for replacing the fan in a MRT Quad nas - does this look like yours?

1) do you have a link to a firmware upgrade?
Do you have access to the serial console? I think JP1 is a MRT compatible serial port. Have a look at the unbricking guide
You could try to use "find" to locate "CheckImage.sh" which checks the firmware (thread at nas-portal)

2) Did you use this disk with this nas before?

4) you may not order a new motherboard. All these nas are manufactured in china and only branded and distributed by some companies.

first of all I buy this nas not working from ebay, for only 40€ and the seller say to me that works before hard disk crash.

This is the problem, when I insert a Disk and try to format it, the operation goes well, the disk is really formatted, but the nas wont's star to open the other function of the nas, here below the screenshot, and also if I try to give command fdisk it seems that no disk is attached, but if i put this disk to another computer the disk is correctly formatted, pheraps the problem is the firmware damage, I wrote to Kraun support to have Kraun NAS firmware (the home page is vary bad), because I try to upload the mrt firmware (because I has already this mrt nas), but the Nas says "wrong id vendor".

yesterday I try a simple operation, change hard disk position on nas bay, and I discover that 2 of 4 bay are damaged.
the 1st and 2nt doesn't works, I mean a nas see hard disk, but command like fdisk or format hasn't effect.